Hi, this might be a case of anthracnose. Is that a cucumber plant?
Cucumber anthracnose is caused by the fungus Colletotrichum orbiculare (C. lagenarium). Look closely for pink spore masses with hair-like structures around the spots (and the undersides). You'll need to remove and destroy infected parts (leaves, stems) to prevent overwintering near the plant and further spread. It spreads via the spores, so you need to be careful with soil reuse and also check the surrounding plants.
With cucumbers you need to start with certified disease-free cucumber seeds and sow them in well-draining soil free of runoff water. Also avoid handling wet cucumber plants to prevent disease spread.
You can apply a fungicide during rainy periods to prevent the fungus "taking". Options include chemical and organic treatment. Consider using potassium bicarbonate, copper-based products, Bacillus subtilis, or horticultural oils.
